In the related art, halogen lamps or high intensity discharge (HID) lamps have been mainly used as light sources for vehicular lamps, in particular headlights, but recently, vehicular lamps using semiconductor light sources such as, for example, light emitting diodes (LEDs) or semiconductor laser diodes (LDs) are being developed instead of such lamps.
A vehicular lamp is mounted with a plurality of light sources of which the on/off is individually controlled. For example, a light source for a low beam and a light source for a high beam may be mounted on the vehicular lamps. FIGS. 1A to 1C are circuit diagrams of a vehicular lamp including a plurality of light sources studied by the present inventors. In each figure, a first light source 302 corresponds to a low beam and a second light source 304 corresponds to a high beam.
A lighting circuit 400R of a vehicular lamp 300R illustrated in FIG. 1A includes a first drive circuit 410 and a second drive circuit 412 that correspond to the first light source 302 and the second light source 304, respectively. Each of the first and second drive circuits 410 and 412 is configured with (i) a constant current output converter or (ii) a combination of a constant voltage output converter and a constant current circuit.
When a power voltage VIN is supplied to an LO terminal, the first drive circuit 410 supplies a drive current (lamp current) ILAMP1 to the first light source 302. When the power voltage VIN is supplied to the LO terminal and a high level is input to an HI terminal, the second drive circuit 412 supplies a drive current ILAMP2 to the second light source 304.
According to the vehicular lamp 300R of FIG. 1A, light sources having different rated currents may be used as the first light source 302 and the second light source 304, but since a drive circuit is required for each light source, the cost becomes high and the size increases.
In a vehicular lamp 300S of FIG. 1B, a lighting circuit 400S includes a common drive circuit 414 for two light sources 302 and 304, and a plurality of constant current circuits 420 and 422. The drive circuit 414 is a constant voltage output converter. The constant current circuit 420 is provided in series with the first light source 302 so as to stabilize the drive current ILAMP1. Also, the constant current circuit 422 is provided in series with the second light source 304, and when the high level is input to the HI terminal, the constant current circuit 422 is turned on to stabilize the drive current ILAMP2.
According to the vehicular lamp 300S of FIG. 1B, since only one drive circuit is sufficient, the cost may be cut down and the size may also be reduced. However, when a difference between a forward voltage VF1 of the first light source 302 and a forward voltage VF2 of the second light source 304 is large, a power consumption (loss) in one of the constant current circuits 420 and 422 increases.
In a vehicular lamp 300T of FIG. 1C, the rated currents of the two light sources 302 and 304 are equal, and these light sources are serially connected. The common drive circuit 414 supplies a common drive current ILAMP to a serially-connected circuit of the light sources 302 and 304. A bypass switch 430 is provided in parallel with the second light source 304, and a switch driver 432 turns off the bypass switch 430 when the HI terminal is at a high level. At this time, the drive current ILAMP is supplied to the second light source 304 so that the second light source 304 is turned on. The switch driver 432 turns on the bypass switch 430 when the HI terminal is at a low level. At this time, the drive current ILAMP flows in the bypass switch 430 so that the second light source 304 is turned off.
In the vehicular lamp 300T of FIG. 1C, since only one drive circuit is sufficient, the cost may be cut down and the size may also be reduced. Further, the problem of power loss as in the constant current circuit illustrated in FIG. 1B does not occur. However, since it is required to select components having similar rated currents for the first light source 302 and the second light source 304, there is a serious design restriction.
Further, although a combination of a high beam and a low beam has been described here, the same problem may occur in a combination of other light sources. See, for example, Japanese Patent Laid-open Publication No. 2006-103404.